1. Field of the Invention
The present invention relates to a process for production of a thin-film transistor (TFT for short hereinafter) with single-crystal semiconductor thin film formed on a substrate having an insulating surface. The present invention relates also to a process for production of a semiconductor device containing semiconductor circuits constructed of TFTs.
The term “semiconductor device”, as used in this specification embraces any device that utilizes semiconductor characteristics for its functions. To be more specific, it includes electro-optical devices typified by liquid crystal displays, semiconductor circuits formed by integration of TFTs, and electronic machines and equipment containing as parts such electro-optical devices and semiconductor circuits.
2. Description of the Relates Art
The recent rapid progress in VLSI technology has yielded SOI (silicon on insulator) which is attracting attention because of its law power consumption. This technology differs from the conventional one in that the bulk single-crystal silicon that forms the active region (or channel-forming region) of FET is replaced by thin-film single-crystal silicon.
An SOI substrate consists of single-crystal silicon and thin film of single-crystal silicon formed thereon, with a buried silicon oxide film interposed between them. There are several known methods for its production. A new technology that is attracting attention is a bonded SOI substrate which is produced by bonding together two silicon substrates. This technology is expected to form thin film of single-crystal silicon on a glass substrate or the like.
The most noticeable among the technologies of bonded SOI substrate is “Smart-Cut” (a registered trademark of SOITEC Co., Ltd.). It utilizes the hydrogen brittleness. For more detail, refer to “Electronic material”, pp. 83-87, August, 1997, issued by Kogyo Chosa kai.
The “Smart-Cut” process consists of forming thin film of single-crystal silicon on a silicon substrate as a support by heat treatment in two steps. The first heat treatment is performed at 400-600° C. so as to bring about hydrogen embrittlement and separate thin film of single-crystal silicon, and the second heat treatment is performed at about 1100° C. so as to stabilize the bonding interface.
The disadvantage of this process is that the second heat treatment causes a strong stress to occur in the thin film of single-crystal silicon and this stress causes the thin film of single-crystal silicon to crack and peel at the edges of the silicon substrate. This is a serious problem with “Smart-Cut” process, and an effective solution to it is required.